Update Auto Test docs about supported buildpacks

See https://gitlab.com/gitlab-org/gitlab/-/issues/222813

doc/topics/autodevops/stages.md

@@ -90,6 +90,12 @@ Check the [currently supported languages](#currently-supported-languages).
 Auto Test uses tests you already have in your application. If there are no
 tests, it's up to you to add them.
 
+NOTE: **Note:**
+Not all buildpacks supported by [Auto Build](#auto-build) are supported by Auto Test.
+Auto Test uses [Herokuish](https://gitlab.com/gitlab-org/gitlab/-/issues/212689), *not*
+Cloud Native Buildpacks, and only buildpacks that implement the
+[Testpack API](https://devcenter.heroku.com/articles/testpack-api) are supported.
+
 ### Currently supported languages
 
 Note that not all buildpacks support Auto Test yet, as it's a relatively new